Recently, with the development of multimedia information, cellular mobile communication systems have been generalized to transmit not only voice data but also mass data such as still image data, moving image data, and the like. In order to realize the transmission of mass data, there has been active study regarding technology using a high frequency radio band to achieve a high transmission rate.
However, in the case of using a high frequency radio band, a high transmission rate can be expected in a short distance, whereas attenuation due to the transmission distance becomes larger as the distance becomes longer. Accordingly, in the case of actually operating a mobile communication system using a high frequency radio band, the coverage area of a wireless communication base station device (hereinafter abbreviated to as a “base station”) becomes smaller, and due to this, there is a need to install many more base stations. Since installing such base stations incurs costs, there has been strong demand for a technology for realizing a communication service using a high frequency radio band while suppressing the increase of the number of base stations.
In order to expand coverage areas of respective base stations with respect to such requests, as shown in a wireless relay system of FIG. 11, a relay transmission technology has been studied, in which a wireless communication relay station device (hereinafter abbreviated to as a “relay station”) 30 is installed between a base station 10 and a wireless communication mobile station device (hereinafter abbreviated to as a “mobile station”) 20, and communications between the base station 10 and the mobile station 20 are performed through the relay station 30. Using the relay technology, a terminal that is unable to communicate directly with the base station 10 can communicate with the base station 10 through the relay station 30. Further, a mobile station is a mobile station that is connected to the base station 10.
[Explanation of TD Relay]
Next, a TD relay will be described with reference to FIGS. 12 and 13. FIG. 12 is a conceptual diagram explaining a TD relay in an uplink, and FIG. 13 is a conceptual diagram explaining a TD relay in a downlink.
In the uplink as shown in FIG. 12, in a subframe #2, the mobile station 20 makes a transmission to the relay station 30 on an access link, and in a subframe #3, the relay station 30 performs communications with the base station 10 on a backhaul link. In a subframe #4, the mobile station 20 makes a transmission again to the relay station 30. Similarly, in the downlink as shown in FIG. 13, in the subframe #2, the relay station 30 makes a transmission to the mobile station 20 on the access link, and in the subframe #3, the base station 10 performs communications with the relay station 30 on the backhaul link. In the subframe #4, the relay station 30 makes a transmission again to the mobile station 20.
As described above, in the TD relay, the transmission from the base station 10 to the relay station 30 and the transmission from the relay station 30 to the mobile station 20 are divided by time. Like this, the communications on the backhaul link and the communications on the access link may be divided by time. The relay station 30 may divide the transmission time and the reception time. Accordingly, the relay station 30 may relay signals without being affected by wraparound between a transmission antenna and a reception antenna.
[Explanation of Guard Time]
In the relay station 30, for switchover from transmission to reception or switchover from reception to transmission, it is necessary to provide a guard period for switching an RF (Radio Frequency) circuit. FIG. 14 is a diagram explaining a guard period. As shown in FIG. 14, for switchover from transmission to reception or switchover from reception to transmission in a subframe, each guard period is provided. The guard period exerts an influence on the performance of the device, and is assumed to be about 20 [μs].
[Switching Timing]
Since the guard period is necessary in the TD relay, a subframe that is unable to be transmitted or received occurs. In the 3GPP RAN1#59b meeting, in order to prepare a guard period, how to sacrifice the first OFDM symbol of a backhaul subframe (hereinafter referred to as “Case A”), how to sacrifice the last OFDM symbol of a backhaul subframe (hereinafter referred to as “Case B”), and how to sacrifice the last OFDM symbol of an access link (hereinafter referred to as “Case C”) have been studied (see NPL 1).
Next, referring to FIGS. 15 to 17, the above-described Case A, Case B, and Case C will be described. FIG. 15 is a diagram explaining an example [1] of preparing a guard period (corresponding to Case A), FIG. 16 is a diagram explaining an example [2] of preparing a guard period (corresponding to Case B), and FIG. 17 is a diagram explaining an example [3] of preparing a guard period (corresponding to Case C). In the drawings, the horizontal axis represents time [μs].
In FIGS. 15 to 17, subframe A (backhaul subframe) indicates communications from a relay station 30 to a base station 10, and subframes B and C (access link subframes) indicate communications from a mobile station 20 to the relay station 30. Each of the subframes A, B, and C includes SC-FDMA (Single Carrier-Frequency Division Multiple Access) symbols #0 to #13. The symbol length of the SC-FDMA symbol is about 71.4 [ms].
Further, in FIGS. 15 to 17, a period between arrows indicated by Δshift B or Δshift F represents a guard period. In the drawings, a switching period indicates time taken for the relay station 30 to perform switching from transmission to reception or switching from reception to transmission.
Further, in FIGS. 15 to 17, rUE transmission indicates a transmission timing of the mobile station 20 to the relay station 30, RN reception indicates a reception timing of the relay station 30 from the mobile station 20, RN transmission indicates the transmission timing of the relay station 30 to the base station 10, and eNB reception indicates the reception timing of the base station 10 from the relay station 30.
Further, in FIGS. 15 to 17, tilting blocks denoted by SC-FDMA symbols #0 to #13 (in the drawings, # is omitted and only numerals are denoted) indicate the propagation delay of symbols. For example, in FIG. 15, the symbols transmitted from the mobile station 20 to the relay station 30 at time T1 are received in the relay station 30 at time T2. Similarly, in FIGS. 16 and 17, the slope of the blocks indicates the propagating delay of the symbols.
The relay station 30 can control the reception timing from the mobile station 20 to the local station (or transmission timing from the mobile station 20 connected to the local station to the local station). Further, the reception timing from the relay station 30 to the base station 10 (or transmission timing from the relay station to the base station 10) has been set in the base station 10.
In the example [1] of preparing the guard period as shown in FIG. 15 (Case A), the relay station 30 transmits SC-FDMA symbols #1 to #13 (in the case of a normal CP length) to the base station 10. The relay station 30 can transmit the SC-FDMA symbol #13. The SC-FDMA symbol #13 is an SRS (Sounding Reference Signal) transmission symbol (defined in the Rel. 8 LTE specification). That is, the relay station 30 can transmit the SRS.
[Explanation of SRS]
The SRS is a signal for measuring the channel quality of a broad band connection. As the mobile station transmits the SRS to the base station, the base station measures the channel quality of an RB through which the SRS has been transmitted and refers to the measured channel quality in order to allocate resources to the mobile station. The SRS is placed in the SC-FDMA symbol #13. In the case of transmitting the SRS, the mobile station transmits the SC-FDMA symbols #0 to #12 on a PUCCH (Physical Uplink Control Channel) and a PUSCH (Physical Uplink Shared Channel), and transmits the SRS on the SC-FDMA symbol #13.
In the example [2] of preparing the guard period as shown in FIG. 16 (Case B), the relay station 30 transmits the SC-FDMA symbols #0 to #12 (in the case of the normal CP length) to the base station 10. In the example shown in FIG. 16, it is possible to apply PUCCH and PDSCH formats of Rel. 8 LTE to the backhaul channel as they are. Further, in the case of using the SC-FDMA 13 for the SRS, an R-PUCCH (PUCCH for a relay) and an R-PUSCH (PUSCH for a relay) use the SC-FDMA 0 to 12.
In the example [3] of preparing the guard period as shown in FIG. 17 (Case C), the relay station 30 transmits the SC-FDMA symbols #0 to #13 (in the case of the normal CP length) to the base station 10. The relay station 30 can transmit all the SC-FDMA symbols, but symbols that the relay station 30 can receive from the mobile station 20 connected to the local station become the SC-FDMA symbols #0 to #12 (in the case of the normal CP).